1. Field of the Invention
The present invention relates to an image display apparatus and a control method therefor.
2. Description of the Related Art
A hold-type image display apparatus, such as a liquid crystal display apparatus (liquid crystal display), incurs a phenomenon called “motion blur” by which a moving object is seen to have tailing in displaying a moving image.
There is a technique for improving the motion blur of such a liquid crystal display apparatus which is called “BL scan” which causes a backlight (BL) to perform impulse-type light emission (by black insertion, or inserting a black image between frames). For example, a technique exists such that in driving a backlight having a plurality of LEDs (light sources) arranged in a matrix form, BL lines of LEDs (matrix lines each formed of a plurality of LEDs) are sequentially lit and sequentially extinguished from the upper side toward the lower side of the screen. If the BL scan is performed only once per frame, a flicker disturbance occurs.
Conventional techniques for reducing the flicker disturbance are disclosed in Japanese Patent Application Laid-open Nos. 2000-322029 and 2008-65228 for example. Specifically, the techniques disclosed in Japanese Patent Application Laid-open Nos. 2000-322029 and 2008-65228 perform a control such as to light the backlight plural times per frame. Further, according to the technique disclosed in Japanese Patent Application Laid-open No. 2008-65228, the backlight is lit with different timings on a frame-to-frame basis.
However, when the techniques disclosed in Japanese Patent Application Laid-open Nos. 2000-322029 and 2008-65228 and the like are used, a double-image blur takes place by which the contour of an object is seen to be multiple. The following description is directed to the motion blur and the double-image blur.
Firstly, the motion blur is described with reference to FIGS. 16A to 16G. FIGS. 16A to 16G are schematic views illustrating an exemplary disturbance (motion blur) which occurs when the image of an object moving on the screen from the left-hand side toward the right-hand side is displayed without the BL scan.
FIG. 16A is a view illustrating an exemplary input image signal (image signal inputted to a liquid crystal display apparatus) which is inputted to a liquid crystal line A (matrix line formed of a plurality of liquid crystal elements) during three frame periods t1, t2 and t3. FIG. 16A illustrates an exemplary image signal indicative of a bright object O moving on a dark background B from the right-hand side toward the left-hand side of the screen.
FIG. 16B is a view illustrating an exemplary transmittance of a liquid crystal element forming the liquid crystal line A during the period t3. The ordinate of FIG. 16B represents the transmittance of the liquid crystal element, while the abscissa of FIG. 16B represents the spatial position (in the horizontal (transverse) direction) of the liquid crystal element. The transmittance corresponds to the brightness of an image.
FIG. 16C is a view illustrating an exemplary vertical sync signal with respect to the input image signal. Each of the periods t1, t2 and t3 is a one-frame period. The vertical sync signal is inputted once per one-frame period.
FIG. 16D is a view illustrating an exemplary lighting state of a backlight (a portion of the backlight corresponding to the liquid crystal line A). The ordinate of FIG. 16D represents time, while the abscissa of FIG. 16D represents the brightness of the backlight at each point in time (instantaneous value, i.e., instantaneous brightness). In FIG. 16D, the instantaneous brightness of the backlight is constantly set to 1.
FIG. 16E is a view illustrating an exemplary display image (image displayed on the screen) displayed on the liquid crystal line A during the three frame periods t1, t2 and t3 described above. The ordinate of FIG. 16E represents time, while the abscissa of FIG. 16E represents the spatial position. Because the backlight is always lit in FIG. 16E (see FIG. 16D), the image based on the input image signal is constantly displayed. In FIG. 16E, only the region of the object O is shown and the region of the background B is not shown.
FIG. 16F is a view illustrating an exemplary integration value of brightness which is inputted to the retinas of the eyes of a viewer, namely, an image perceived by the viewer (image on the liquid crystal line A) when the eyes of the viewer (user) follow the object O moving.
FIG. 16G is a view illustrating a distribution of the integration value shown in FIG. 16F (i.e., brightness distribution). When FIGS. 16B and 16G are compared to each other, the brightness of an edge portion of the object O changes steeply in FIG. 16B, whereas the brightness of an edge portion 1501 of the object O changes gently in FIG. 16G. This means that a blur (motion blur) occurs at the edge portion of the object O.
The next description is directed to the double-image blur with reference to FIGS. 17A to 17G. FIGS. 17A to 17G are schematic views illustrating an exemplary disturbance (including the motion blur and the double-image blur) which occurs when the image of an object moving on the screen from the left-hand side toward the right-hand side is displayed while the BL scan as disclosed in Japanese Patent Application Laid-open Nos. 2000-322029 and 2008-65228 is performed.
FIGS. 17A to 17C are identical with FIGS. 16A to 16C, respectively.
FIG. 17D is a view illustrating an exemplary lighting state of a backlight (a portion of the backlight corresponding to the liquid crystal line A). The ordinate of FIG. 17D represents time, while the abscissa of FIG. 17D represents the instantaneous brightness of the backlight at each point in time. In FIG. 17D, two lighting periods of the backlight are provided within one frame. The instantaneous brightness of the backlight in each lighting period is constantly set to 2. This is done in order to maintain the total amount of light emitted from the backlight during one frame.
FIG. 17E is an exemplary display image displayed on the liquid crystal line A during the three frame periods t1, t2 and t3. The ordinate of FIG. 17E represents time, while the abscissa of FIG. 17E represents the spatial position. In FIG. 17E, an image based on an input image signal is displayed during the lighting periods of the backlight (however, the brightness of the image is higher than in FIG. 16E), while a black image is displayed during non-lighting periods (extinction periods) of the backlight. This means that the image based on the input image signal and the black image are displayed alternately. In FIG. 17E, only the region of the object O is shown and the region of the background B is not shown.
FIG. 17F is a view illustrating an exemplary integration value of brightness which is inputted to the retinas of the eyes of a viewer, namely, an image perceived by the viewer (image on the liquid crystal line A) when the eyes of the viewer follow the object O moving.
FIG. 17G is a view illustrating a distribution of the integration value shown in FIG. 17F (i.e., brightness distribution). The change in the brightness of an edge portion 1601 of the object O is steeper in FIG. 17G than in FIG. 16G. This means that the blur (motion blur) that occurs at the edge portion of the object O is improved. In the example shown in FIG. 17G, however, the change in the brightness of the edge portion 1601 contains a flat portion 1602 which is a region in which the brightness stays constant. The brightness of a flat portion 1602 is a value at substantially the midpoint between the brightness of the background B and that of the object O. Such a flat portion brings about the double-image blur.
By performing only the BL scan disclosed in Japanese Patent Application Laid-open Nos. 2000-322029 and 2008-65228, the flicker disturbance and the motion blur can be reduced, but the double-image blur is allowed to occur.
A conventional technique for reducing such a double-image blur is disclosed in Japanese Patent Application Laid-open No. 2006-18200 for example. Specifically, the technique disclosed in Japanese Patent Application Laid-open No. 2006-18200 uses a lighting signal (backlight drive signal) which is the OR of a pulse signal given once per frame and a pulse signal given with a higher frequency than the frame frequency. The technique disclosed in Japanese Patent Application Laid-open No. 2006-18200 reduces the double-image blur by using such a lighting signal.
However, some display images relying upon the above-described techniques disclosed in Japanese Patent Application Laid-open Nos. 2000-322029, 2008-65228 and 2006-18200 allow the flicker disturbance to be visually observed because the number of times of lighting of the backlight within one frame is constant.